Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the blood oxygen saturation, serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heart beat.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmissivity and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined.
Conventional pulse oximeters for measuring the heart rate and the oxygen saturation of a subject are attached to the skin of the subject, for instance to a finger tip, earlobe or forehead. Therefore, they are referred to as ‘contact’ PPG devices. A typical pulse oximeter comprises a red LED and an infrared LED as light sources and one photodiode for detecting light that has been transmitted through patient tissue. Commercially available pulse oximeters quickly switch between measurements at a red and an infrared wavelength and thereby measure the transmissivity of the same area or volume of tissue at two different wavelengths. This is referred to as time-division-multiplexing. The transmissivity over time at each wavelength gives the PPG waveforms (also called PPG signals) for red and infrared wavelengths. Although contact PPG is regarded as a basically non-invasive technique, contact PPG measurement is often experienced as being unpleasant, since the pulse oximeter is directly attached to the subject and any cables limit the freedom to move.
Recently, non-contact, remote PPG devices for unobtrusive measurements have been introduced. Remote PPG utilizes light sources or, in general radiation sources, disposed remotely from the subject of interest. Similarly, also a detector, e.g., a camera or a photo detector, can be disposed remotely from the subject of interest. Therefore, remote photoplethysmographic systems and devices are considered unobtrusive and well suited for medical as well as non-medical everyday applications.
Wieringa, et al., “Contactless Multiple Wavelength Photoplethysmographic Imaging: A First Step Toward “SpO2 Camera” Technology,” Ann. Biomed. Eng. 33, 1034-1041 (2005), discloses a remote PPG system for contactless imaging of arterial blood oxygen saturation in tissue based upon the measurement of plethysmographic signals at different wavelengths. The system comprises a monochrome CMOS-camera and a light source with LEDs of three different wavelengths. The camera sequentially acquires three movies of the subject. During each movie, the subject is illuminated by the light source at a different wavelength. The pulse rate can be determined from a movie at a single wavelength, whereas at least two movies at different wavelengths are required for determining the oxygen saturation. The measurements are performed in a darkroom, using only one wavelength at a time. The patient is not allowed to move between the subsequent measurements at different wavelengths. A further problem is that a measurement in darkness is not practical for unobtrusive medical and non-medical applications.
Toth B. et al.: “Oxygen saturation in healthy newborn infants immediately after birth measured by pulse oximetry”, Archives of Gynecology and Obstetrics, Springer Verlag, Berlin, DE, Vol. 266, No. 2, 1 Apr. 2002, pages 105-107 discloses a study of the measurement of pre- and postductal arterial oxygen saturation rates that were measured in 50 healthy newborn infants. Two pulse oximeters were used to measure SpO2 continuously with sensors placed around the ulna side of the right wrist and around the right midfoot, representing pre- and postductal SpO2 value respectively. It was found that neonates need approximately 15 min to reach adult SpO2 values.
DE 10 2010 056 615 A1 discloses a camera-based transillumination measurement device for contactless space-resolved vital parameter determination of neonates from camera images.
US 2011/0311143 A1 discloses a method of controlling a function of a device and system for detecting the presence of a living being.
WO 2013/017976 A1 discloses a device for obtaining an processing measurement readings including at least a component representative of a physical phenomenon in a living being, comprising a sensor for obtaining measurement readings from at least one body part of a living being from a distance having at least a component representative of the physical phenomenon in the living being, an identification unit for identifying the at least one body part of the living being; an extraction unit for extracting at least one first signal from the measurement readings representing at least one component representative of the physical phenomenon, an evaluation unit for obtaining adjustment information according to the at least one identified body part, and an adjustment unit for adjusting the at least one first signal according to the adjustment information and for generating at least one output signal representing the physical phenomenon of the living being.